Electron spin mechanisms for inducing magnetic-polarization reversal

ABSTRACT

An apparatus includes a low magnetic-coercivity layer of material (LMC layer) having a majority electron-spin-polarization (M-ESP), an energy-gap coupled with the LMC layer, wherein a flow of spin-polarized electrons having an electron-spin-polarization anti-parallel to the LMC layer are injected via the energy-gap, to change the M-ESP of the LMC layer. A non-magnetic material is in electrical communication with the LMC layer and provides a spin-balanced source of current to the LMC layer, responsive to the injection of spin-polarized electrons into the LMC layer.

This application is a divisional of application Ser. No. 09/967,170,filed on Sep. 27, 2001, now U.S. Pat. No. 6,741,496

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates generally to magnetic-polarization control andmore specifically to magnetic memory systems enhanced through thecontrol of spin-polarized electrons.

2. Art Background

Present transistor-capacitor based metal oxide semiconductor (MOS) andcomplimentary metal oxide semiconductor (CMOS) non-volatile memorytechnologies are approaching practical memory density limits as thelithography and material processes have been scaled down to smallergeometries. The oxide thickness of flash memory cells cannot be mademuch thinner with existing materials without allowing the undesirablecondition of hot electron tunneling to occur. Thus, a practical memorydensity limit is approaching for traditional transistor basednon-volatile memory devices.

Electron spin based devices are being used as memory cells for thestorage of data. For example, magnetic random access memory (MRAM)offers the possibility of replacing flash memory technology with a lowervoltage, scalable technology. The spins of electrons are tied tomagnetism, as in the ensemble of spins in a memory cell or quantum welldevice.

Presently constructed MRAM uses giant magneto-resistance (GMR) ormagnetic tunnel junctions (MTJ) to control the sense/tunneling currents.These devices manipulate the magnetic state of the memory cell by usingthe coupling of strong magnetic fields induced by currents in conductorsthat are proximate with and magnetically coupled to the magnetic memorycell. Randomly polarized electrons conducted through these conductorsare used to induce a strong magnetic field that causes reversing ofopposing magnetic domains. This process requires a very high currentdensity and dissipates large amounts of energy, hence it is inefficientand inconsistent with the requirements of scalable, low power memory.

The basic GMR memory cell uses a three-layer composite consisting of aweaker magnetic layer/non-magnetic conductor layer/stronger referencemagnetic layer. The change in impedance across the cell varies betweenthe two memory states corresponding to the alignment of themagnetic-polarizations of the two magnetic layers being aligned oranti-aligned. A larger impedance change between the aligned andanti-aligned states corresponds to a greater detectable signal level.Therefore, magnetic memory cells could be made smaller if the impedancechange could be increased.

What is needed is a memory device that permits its magnetic-polarizationto be changed using less power while providing a greater detectablesignal. What is also needed is a magnetic memory device that isconfigurable in higher memory densities than are presently achievablewith conventional flash memory architecture or present MRAM memorycells.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited in the figures of the accompanying drawings, in which likereferences indicate similar elements.

FIG. 1 illustrates magnetic polarization reversal within a magneticmaterial.

FIG. 1B illustrates an energy distribution as a function of wavenumberfor magnetic materials of opposite polarity, a non-magnetic material,and a half-metallic material.

FIG. 2 is a plot of scattering probability for opposite and same spinelectrons as a function of included scattering angle.

FIG. 3 is a plot of a mean-free-path of an injected electron beforecollision with another electron as a function of energy level of theinjected electron.

FIG. 4 is a critical equation governing reversal of an electron-spinpolarization within a magnetic volume.

FIG. 5 is an illustration of magnetic gain using a tunnel junction.

FIG. 6 illustrates a magnetic phase behavior of a magnetic storage layerand a half-metallic layer.

FIG. 7 illustrates a phase behavior between a storage layer and a highlypolarized weakly coupled magnetic layer.

FIG. 8 illustrates a majority electron-spin-polarization of a lowmagnetic-coercivity layer (LMC) storage layer and a highmagnetic-coercivity layer (HMC) reference layer for two different memorystates of a typical memory cell.

FIG. 8B illustrates a dual magnetic-mirror/conductor junction.

FIG. 9 depicts a schematic representation of a vertical magnetic randomaccess memory (VMRAM) memory cell.

FIG. 10 illustrates a cross sectional view of a vertical magnetic randomaccess memory (VMRAM) memory cell.

DETAILED DESCRIPTION

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings in which like referencesindicate similar elements, and in which is shown by way of illustration,specific embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. The following detailed descriptionis, therefore, not to be taken in a limiting sense, and the scope of theinvention is defined only by the appended claims.

A low power method and apparatus for reversing a majorityelectron-spin-polarization (M-ESP) of a magnetic material is disclosed.In one embodiment of the present invention, magnetic polarizationreversal within a magnetic material is illustrated in FIG. 1 at 10. Withreference to FIG. 1, a magnetic material 4 will undergo reversal of itsM-ESP subsequent to injection of a flow of spin-polarized (SP) electronswhose electron-spin-polarization (ESP) is anti-parallel to the M-ESP ofthe magnetic material 4.

A magnetic material's magnetic polarization may be described as beingeither parallel to e1 or e2. e1 and e2 are electron spin polarizationswhich will be described below in conjunction with FIG. 1B. The magneticmaterial 4 will be arbitrarily assigned a M-ESP of e2. A typicalmagnetic material for use in this application is permaloy ornickel-cobalt-iron.

A normal conductor 8 is coupled with the magnetic material 4 as shown inFIG. 1. The normal conductor can be a metal such as copper, or any othercombination of materials that provide a spin-balanced current (equalnumbers of e1 and e2 electrons described below in conjunction with FIG.1B) in a low work function material. Additionally, a normal conductorexhibits conductive behavior at 300 degrees Centigrade.

An energy-gap 2 is coupled with the magnetic material 4. In oneembodiment of the present invention, the energy-gap 2 may be a tunneljunction layer of material (a tunnel junction is describe more fullybelow in conjunction with FIG. 5). A typical tunnel junction can be alayer of an oxide such as aluminum oxide or titanium oxide. A typicalthickness of a tunnel junction layer is between one and twenty angstroms(nM). In an alternative embodiment of the present invention, theenergy-gap 4 can be constructed with two magnetic-mirrors separated by anormal conductor. In one embodiment, a magnetic-mirror can be ahalf-metallic layer of material such as chromium dioxide. Magneticmirrors and half-metallic material will be described below inconjunction with FIGS. 6-8. The energy-gap 4 has the property of anelectrical insulator and performs the function of magneticallydecoupling magnetic materials in contact with adjacent sides of theenergy-gap. Electrons may be injected across or via an energy-gapwithout dissipation of energy from resistance. Thus, the energy-gapprovides a resistance free barrier by which high-energy electrons 12 maybe injected into the magnetic material.

For the purpose of this example, since the M-ESP of the magneticmaterial 4 was arbitrarily assigned the value of e2, thespin-polarization of the injected high-energy electrons must be ofopposite spin-polarity to ensure preferential collisions with majorityspin electrons 14 consistent with the Pauli Exclusion Principle(described below in conjunction with FIG. 2). The injected high-energye1-SP electrons 12 a collide with electrons of opposite ESP 14, in thisexample e2-SP due to the M-ESP of the material being chosen as e2.

Several mechanisms for achieving magnetic gain are taught according tovarious embodiments of the present invention. Magnetic gain is usefulfor reversing the polarization state of the magnetic material 4. As waspreviously mentioned, the present invention teaches low power methodsfor reversing the magnetic polarization state of a magnetic material.Thus, magnetic-gain reduces the required power necessary forpolarization reversal, and will be described more fully below inconjunction with FIGS. 4-8B.

With reference to FIG. 1, magnetic gain is illustrated at 20. Aninjected high-energy e1-SP electron at 26 interacts with a plurality ofe2-SP electrons in magnetic material 4, which are at an energy levelapproximately equal to thermal energy, exciting or scatteringapproximately one hundred (100) e2-SP electrons as shown at 28. The gainassociated with one high-energy e1-SP electron 26 causing one hundred(100) e2-SP electrons to leave material 4 is described more fully belowin conjunction with FIG. 5. As just described, e2-SP electrons 28 areexcited or scattered and are driven out of the magnetic material 4 (asdescribed below in conjunction with FIG. 3) interacting with normalconductor 8 to provide a source of magnetic gain as illustrated in 20 inFIG. 1. As e2-SP electrons leave the magnetic material 4 passing throughthe normal conductor 8, an induced electrostatic potential arises thatcauses a spin-balanced current to flow back into the magnetic material 4supplying approximately fifty (50) e1-SP electrons 30 for each injectede1-SP electron 26. Additionally, magnetic gain can be achieved bycreating a magnetic-mirror. A magnetic-mirror allows electrons with anESP parallel to the ESP of the mirror to pass through the mirror butcauses electrons with an ESP anti-parallel to the mirror to accumulateand interact with the magnetic material, thereby furthering the reversalof the M-ESP of the magnetic material. In one embodiment, amagnetic-mirror can be a half-metallic layer of material such aschromium dioxide. Use of a magnetic-mirror is described more fully belowin conjunction with FIGS. 6-8. Thus, magnetic gain is achieved with thepresent invention according to several different mechanisms.

As the population of e1-SP electrons grows to the detriment of the e2-SPelectrons the M-ESP of the material is overwhelmed and is reversed bythe present invention. Magnetic gain serves to decrease the requiredpower consistent with the requirement of overcoming thermal relaxationwithin the material as described more fully in conjunction with FIG. 4below. Thermal relaxation requires the reversal of the M-ESP of thematerial to be achieved within approximately 1 nanosecond (ns) for theexample of Iron (Fe) described in FIG. 4.

An application of the present invention is taught, in one embodiment, toa magnetic memory cell that is capable of being operated between twologic states. The logic states correspond to two different majorityelectron-spin-polarizations of electrons within a magnetic storage layerof low magnetic-coercivity (LMC) material. LMC material is used for thestorage layer so that the M-ESP of the storage layer may be reversedwithout affecting the M-ESP of the reference layer of the memory cell. Atypical material to use for making the storage layer is permaloy. Themagnetic memory cell uses a reference layer of high magnetic-coercivity(HMC) material that does not change M-ESP. A typical material to use formaking the reference layer is alnico. However, it will be noted by thoseskilled in the art that the same type of material may be used for boththe LMC and the HMC layer, such as nickel-cobalt or nickel-cobalt-iron.It is generally desirable to use a system of materials with similaratomic spacing so that a row of atoms is not pressed out of existence bydissimilar atomic spacing which can create gaps and undesirable leakagepaths in the atomic lattice.

A logic state of the memory cell is sensed by measuring the combinedimpedance of the reference magnetic layer, the insulation layer, and thestorage magnetic layer. The impedance of the cell changes when thestorage layer's M-ESP changes relative to the fixed state of thereference layer, thus describing two memory states. In one embodiment ofthe present invention, the M-ESP of the storage magnetic layer ischanged by injecting electrons of opposite electron-spin-polarization(ESP) into the storage layer through an energy-gap which may be a tunneljunction layer or a normal conductor disposed between twomagnetic-mirrors. Injecting high-energy SP electrons through anenergy-gap allows electrons to be injected at high-energy and lowcurrent, which accomplishes reversal of the magnetic polarizationaccording to the teachings of the present invention.

Reversal of the magnetic polarity of a magnetic material, i.e., theM-ESP, according to the embodiments of the present invention, beginswith a combination of material selected and built to achieve theparticular magnetic properties previously described in conjunction withFIG. 1. Magnetic properties of materials are described with reference tothe spin-polarization of electrons contained within the material.

Several ways of describing the spin-polarization of an electron havebeen used by those skilled in the art, such as spin-up and spin-down.Electron spin is described by a quantum number m and takes on values of−½ and +½, corresponding to the two unique directions of spin anelectron may assume. The electron can line up parallel (spin-up) oranti-parallel (spin-down) to an atom's magnetic field. The electron'sspin makes it act as a tiny magnet. The terms spin-up and spin-down alsocorrespond to clockwise or counterclockwise rotation. To simplifynomenclature herein the term spin-up will correspond to the symbol e1and the term spin-down will correspond to the symbol e2. Technically,the spin direction is referenced to an arbitrarily defined coordinatesystem, where a positive direction on the wavenumber axis 106, used inFIG. 1B, corresponds to clockwise rotation and the negative direction onthe wavenumber axis 106 corresponds to counterclockwise rotation. Analternate description of the definition (reversing the association ofcounterclockwise and clockwise with the wavenumber axis) would beequivalent. The labels used to describe the property i.e., “spin-up” and“spin-down” are not limiting to the present invention. What is importantis that there are two polarizations of electrons that may be used tomanipulate the magnetic state of a layer of material.

With reference to FIG. 1B, the energy distribution of a magneticmaterial is shown at 100. Energy level 108 is plotted as a function ofwavenumber 106. The material represented by 100 has electrons spinningin polarization e1 with energy distribution 102 and in polarization e2with energy distribution 104. The exchange energy lowers the energy ofthe electrons with the majority spin direction in a magnetic metal. Theexchange energy is about two (2) electron volts (eV) in typical magneticmetals. The exchange energy creates a split band model. The overallmagnetization of the metal with a split band is in the direction of themajority of the spins. The magnetic material 100 is dominated byelectrons with a M-ESP of e1. A magnetic material has both energydistributions 104 and 102 at Fermi level 110 as shown in 100. Anelectron current may be drawn from a magnetic material, which will bedominated by electrons with the majority spin polarization of themagnetic material.

A magnetic material of opposite polarization is shown at 125. Energydistribution 132 is plotted against wavenumber 130. An energydistribution 126 for e1 electrons and an energy distribution 128 for e2electrons is illustrated in 125. The magnetic material 125 has an M-ESPof e2.

An energy distribution for a non-magnetic metal may be modeled as shownat 150. The electrons fill up the bands to a highest energy level calledthe Fermi level. Since there are equal numbers of e1 and e2spin-polarized (SP) electrons this metal is non-magnetic. Currentflowing from a non-magnetic metal will have equal numbers of e1-SP ande2-SP electrons and will be referred to as a spin-balanced current ofelectrons in a subsequent discussion.

An energy distribution of a magnetic-mirror is shown at 175, with energylevel 182 plotted against wavenumber 180. The e2-SP electron energydistribution 178 reaches a Fermi level 184. However an e1-SP electron'senergy distribution at 176 does not reach the Fermi level 184. Thiscondition within the magnetic-mirror, illustrated at 175, causes e1-SPelectrons to be reflected, while e2-SP electrons may penetrate themagnetic-mirror since the energy levels for the e2-SP electrons reachthe Fermi level 184. The magnetic-mirror acts as a mirror tominority-spin electrons. The strength of the magnetic-mirror depends onthe particular configuration. In one embodiment, a half-metallic willfunction as a magnetic-mirror.

A half-metallic material contains a very high magnetic polarization.Typically, a half-metallic material exhibits magnetic polarization inexcess of 90%. Therefore, a half-metallic, polarized as shown in 175,would allow e2-SP electrons to pass through it, while stalling andaccumulating e1-SP electrons (minority spin electrons). The presentinvention exploits this property of half-metallics to achieve reversalof the M-ESP of a magnetic layer magnetically coupled to thehalf-metallic layer. A value of 0.7 electron volt (eV) is typicallyneeded to overcome a half-metallic's gap potential by electrons ofopposite ESP (minority spin electrons).

Spin-Polarized Scattering Probabilities and Mean Free Path

Electrons having an electron-spin-polarization (ESP) anti-parallel tothe majority electron-spin-polarization (M-ESP) of the magnetic materialare chosen for injection because of the preferred scattering that occursdue to the anti-parallel condition. A probability of scattering betweentwo electrons depends on both electron's spin polarizations i.e., e1-ESPand e2-ESP. FIG. 2 is a plot of a scattering probability for oppositeand same spin electrons as a function of included scattering angle. Withreference to FIG. 2, 200 shows the scattering probability distribution202 as a function of scattering angle for two electrons colliding withopposite spin, i.e., one electron has e1 spin-polarization and the otherhas e2 spin-polarization. Scattering probability 206 is plotted as afunction of the angle formed by the trajectories of the electrons beforecollision.

A scattering probability distribution 252 is plotted in 250 forelectrons having the same spin polarizations (i.e., e1&e1 or e2&e2).Here, as in 200, scattering probability 256 is plotted as a function ofthe angle formed by the trajectories of the electrons before collision.Two same spin electrons approaching each other at 1.5 radians (approx.90 degrees) have a zero probability of scattering (colliding) accordingto 252. An integration of the scattering probability distributions 202and 252 across scattering angle show that the scattering probability ofopposite spin electrons colliding is greater than same spin electronscolliding. Thus, opposite spin electrons will collide more readily thansame spin electrons. This behavior of electron scattering results fromthe Pauli Exclusion Principle and will be used to exploit reversing orspin-flipping a magnetic-polarization state of a data storage layer in amemory device, as described in the upcoming sections.

FIG. 3, is a plot of the distribution 304 of a mean free path of anelectron before collision 302 with another electron as a function ofenergy level of the injected electron 306. From FIG. 3 it is observedthat electrons injected at a high energy level travel a short distancebefore colliding with another electron. From the previous discussion ofFIG. 2, the probability of collision is in favor of a collision betweenelectrons of opposite spin polarization. Therefore, if the M-ESP of amaterial was parallel to the e2 polarization and e1-SP high-energyelectrons were injected into the material, collisions would more readilyoccur with e2-SP low-energy electrons than with e1-SP low-energyelectrons. The e2-SP electrons excited by the high-energy injected e1-SPelectrons have much less energy than the 1 eV injection energy, onaverage. These excited e2-SP electrons will travel a large distancebefore suffering a collision with another electron (FIG. 3). Thiscombination of scattering mechanisms constitutes a very efficientmajority-spin pump for removing e2-SP electrons from the magneticvolume. It will be appreciated by those skilled in the art that the sameresult is achieved if the M-ESP of the material were parallel with thee1-ESP and high-energy e2-SP electrons were injected with the intent offlipping the M-ESP of the material from the e1-ESP to the e2-ESP.

Relaxation Time

In order to achieve reversal of the magnetic polarization of themagnetic material it is necessary to inject a sufficient quantity ofanti-parallel spin-polarized (SP) electrons in a time interval smallerthan the relaxation time of the material. A ferromagnetic materialmaintains its magnetic-polarization because to do so is consistent withthe laws of thermodynamics and allows the material to maintain a stateof minimum energy. As e1-SP electrons are injected into the materialdescribed previously, having e2 M-ESP, they will be opposing thedominant exchange energy that creates and sustains ferromagnetism. Thenewly injected e1-SP electrons will become thermally agitated and willbe induced to flip their spin with the creation of magnons, phonons, andinelastically scattered electrons inside a magnetic lattice. A magneticvolume of material will tend to restore the status quo (e2-ESP, withinthe example arbitrarily chosen herein) within itself by relaxation.

To overcome relaxation, a sufficient amount of e1-SP electrons must betransferred to the magnetic volume within a time less than the timerequired to relax the e1-ESP to e2-ESP. The critical equation governingthe parameters involved in determining spin-flip is shown in FIG. 4 at402. With reference to FIG. 4, the rate of thermal relaxation for themagnetic volume may be determined by the methods of ElectronParamagnetic Resonance and related techniques as are known in the art. Areasonable spin-relaxation time 406 for typical materials such aspermaloy, used for the magnetic storage layer, are on the order of 1nanosecond (ns). A number of spins in a magnetic volume 404 may becalculated by assuming a magnetic bit size measuring 40 nanometers (nM)on a side, containing 3 monolayers of Iron (Fe) atoms. Fe has aconcentration of 8.5×10²² atoms/cm³ and a nearest neighbor spacing of2.48×10⁻¹⁰ meters. Thus, each bit consists of 1.01×10⁵ atoms, the valueof 404 in FIG. 4. This implies that the magnetic bit is relaxingapproximately 1×10¹⁴ spins/sec. If a magnetic gain 412 were unity, anelectron current 408 of 16 micro Amps (μA) would be required to flip theM-ESP of the bit from e2-ESP to e1-ESP. An electron unit charge 410 hasa value of −1.602×10⁻¹⁹ coulomb. Magnetic gain 412 lowers the requiredelectron current 408 for a given spin-relaxation time 406.

Magnetic Gain

Magnetic gain lowers the power required to reverse the magneticpolarization of a material for a given spin-relaxation time. Severalsources of magnetic gain are available according to various embodimentof the invention. In one embodiment, magnetic gain is achieved by arelative difference between an energy level of an electron injectioncurrent and an energy level of the magnetic volume at equilibrium. FIG.5 is an illustration of magnetic gain using a tunnel junction. Withreference to FIG. 5, a low magnetic-coercivity layer of material isshown at 504. Low magnetic-coercivity (LMC) implies that the M-ESP of amaterial may be influenced and is less resistant to change than amaterial of high magnetic-coercivity. A non-exclusive list of materialsthat may be used for the LMC layer includes permaloy,nickel-cobalt-iron, iron-cobalt, and chromium dioxide. A non-exclusivelist of materials that may be used for the HMC layer includes alnico,nickel-cobalt-iron, and nickel-cobalt. Consistent with the presentexample used herein, a M-ESP of 504 is parallel to the e2-ESP. Electronsat e1-ESP 512 are being injected across a tunnel junction (TJ) 502. TJ502 has a property of an electrical insulator as well as performing afunction of magnetically decoupling magnetic layer 504 and a secondadjacent magnetic reference layer (not shown). The material used for TJ502 may be selected for an appropriate atomic spacing to match themagnetic material in contact therewith. Materials such as aluminumoxide, titanium oxide or tungsten oxide may be used for TJ 502. Thethickness of the TJ layer is subject to the details of the particulardesign, thicknesses of one to twenty angstroms (nM) are typical.However, the present invention is not limited thereby.

Electrons 512 may be raised to a potential of approximately one (1)electron volt (eV) by electromotive force 516 and 515, resistor 518 andpath 517 across TJ 502. Resistor 518 is selected to have a value ofseveral thousand ohms (Kohms) greater than the maximum tunnel junctionresistance. An energy level within LMC 504 at equilibrium corresponds tothermal energy, which is approximately 25 milielectron volts (meV) atroom temperature. Assuming equal energy transfer through the volume of504, approximately one hundred (100) e2-SP electrons 514 should bedriven out of LMC 504 by each injected high-energy e1-SP electron 512 a.The e2-SP electrons driven out by collision with e1-SP electrons passinto normal conductor 508 along a plurality of possible trajectories,one such trajectory is shown at 514 a. Some of the injected high-energyelectrons 512 a also pass out of LMC 504 along a plurality oftrajectories; one such trajectory is shown at 512 b.

As e2-SP electrons leave LMC 504 an induced electrostatic potentialarises that causes a spin-balanced current to flow back into LMC 504from normal conductor 508. Normal conductor 508 is non-magnetic, and hasan equal amount of both e2-SP and e1-SP electrons as shown in 150 (FIG.1B). Thus, the induced current flow from 508 to 504 is spin-balanced,meaning equal numbers of e2-SP and e1-SP electrons return to LMC 504. Atequilibrium and charge balance, the non-magnetic metal, normal conductor508, will have replaced on average each of two e2-SP electrons that leftLMC 504 with one e2-SP and one e1-SP electron. The result will be asystem gain of ½ of an e1-SP electron per e2-SP electron that wasenergized, leaving LMC 504. Integrating these changing spin populationsover time leads to a steady growth of e1-SP over e2-SP electrons thateventually result in a reversal of the M-ESP of LMC 504. The reversedmagnetic polarity of LMC 504 is parallel with the e1-SP.

The previously described effect is illustrated in FIG. 5 at 520. LMC isshown at 522 with e1-SP electrons being injected at 526. e2-SP electronsare driven off at 528 with the associated induced current flow ofspin-balanced current 530 from normal conductor 524 to LMC 522. Thus, itis not detrimental that injected e1-SP electrons also leave LMC 504 asshown by path 512 b since e1-SP electrons are replenished by the inducedspin-balanced current flow 530. As previously mentioned, a magnetic gainof one hundred (100) is a reasonable estimate for 412 in FIG. 4,according to the magnetic gain mechanisms described herein which wouldresult in lowering injection current 408 to approximately 100 nanoamps(nA) (FIG. 4).

In one embodiment, a normal conductor 519 (FIG. 5) may be placed betweenTJ 502 and LMC 504. Additionally in this embodiment, path 519 a can beconnected with resistor 519 b back to the conductor 519. Path 517 andresistor 518 may be removed.

Magnetic-Mirror

In several embodiments, magnetic gain may be achieved by the use of amagnetic-mirror layer that acts as a mirror, reflecting incidentopposite spin electrons, while allowing same spin electrons to passthrough. In one embodiment, FIG. 6 illustrates the magnetic phasebehavior of the magnetic storage layer and a half-metallic layer. In oneembodiment, the magnetic-mirror may be a half-metallic layer. Withreference to FIG. 6, half-metallic layer (ML) 652 is disposed beneaththe LMC layer 504. A non-exclusive list of material that thehalf-metallic layer may be made from includes chromium dioxide, titaniumdioxide, or rubidium dioxide. The thickness of the half-metallic layeris often in the nanometer range, typically less than 10 nanometers andis based on the material chosen. Returning to the discussion of flippingthe M-ESP of the LMC layer 504, injected high-energy e1-SP electronsenter 504, as shown at 512 a colliding with opposite spin e2-SPelectrons 514 that are scattered. The scattered e2-SP 514 electronsfollow a plurality of paths subsequent to scattering. One such path isshown at 514 a, where a scattered e2-SP electron passes through the MLlayer 652. The ML layer has energy distributions with respect to eachelectron-spin-polarization (ESP) as shown in 175 (FIG. 1B). Withreference to FIG. 1B, since only the e2-SP energy states 178 reach theFermi level 184, ML 652 will be receptive to e2-SP electrons. The e1-SPenergy states 176 do not reach the Fermi level 178, therefore e1-SPelectrons are reflected off of the ML layer. The ML layer acts like amagnetic insulator to e1-SP electrons when its present ESP is parallelto LMC 504 (e2-ESP). Thus, the half-metallic layer causes e1-SPelectrons to accumulate within the LMC 504 layer, which aids reversal ofthe magnetic polarization (M-ESP) of the LMC 504 layer.

With reference back to FIG. 6, the M-ESP of LMC 504 and the ESP of ML652 are shown at 600 for one cycle of flipping the magnetic polarityfrom e2-SP to e1-SP and then back to e2-SP. Injected high-energy e1-SPelectrons 512 a, previously described, are injected for approximately ananosecond 612 causing the M-ESP of LMC 504 to flip from e2-SP at 606 ato e1-SP at 608. The M-ESP of the LMC and ML layers remain in phase witheach other as illustrated at 628. Time is plotted for the LMC layer at602 and is plotted similarly for the ML layer at 618. There is a verysmall delay (not shown) in the response of the ML layer, which is on theorder of less than a picosecond (1×10⁻¹² seconds). It should be notedthat the orientation of the energy states shown in FIG. 1B at 175corresponds to the ESP of the ML at 622 and 630 in FIG. 6. During ESP624, the energy distributions shown at 175 in FIG. 1B would be mirroredwith respect to the zero of the wavenumber axis 180, where e1-SP stateswould reach the Fermi level 184 and e2-SP states would not be filled tothe Fermi level (not shown). Thus, the ML 652 layer's ESP is dominatedby the M-ESP of LMC 504 and acts as a magnetic-mirror which flips ESPwith the flip of the M-ESP of LMC 504.

To flip the M-ESP of the LMC 504 layer back to e2-ESP, e2-SP high-energyelectrons 616 are injected for approximately a nanosecond at 614 causingthe M-ESP of the LMC layer to reverse from e1-ESP at 608 to e2-ESP at610. The ML layer flips ESP from e1-ESP at 624 to e2-ESP at 630,following the LMC layer as previously described.

The two magnetic polarities may be used as memory states correspondingto “0” and “1.” M-ESP e2 has been arbitrarily assigned to memory state“1” and M-ESP e1 has been arbitrarily assigned to memory state “0.” Itwill be noted by those skilled in the art the opposite assignment ofmemory states with M-ESP could have been made. The invention is notlimited by the assignment chosen.

Highly Polarized Weakly Coupled Magnetic Layer

According to another embodiment, a magnetic-mirror may be made using anisolation barrier and highly polarized magnetic layer to providereflection of injected high-energy electrons. An isolation barrier canbe made from a normal conductor such as copper. With reference to FIG.7, the phase behavior between the LMC 504 layer and a highly polarizedweakly coupled magnetic layer is illustrated. LMC 504 layer is shown incontact with isolation barrier (IB) 702 and highly polarized weaklycoupled magnetic layer (HPML) 704. The M-ESP of the HPML 704 layer isdominated by the M-ESP of the LMC 504 layer. The HPML layer's responsefollows the polarization reversal of the LMC 504 layer. However, theHPML response is delayed in time by time delay 714 as transition frome2-ESP to e1-ESP occurs and time delay 718 as transition from e1-ESP toe2-ESP occurs.

In FIG. 7, LMC M-ESP 604 plotted with time 602 is the same as shown inFIG. 6. HPML M-ESP state 708 is plotted with time 706. The HPML M-ESPstate at 710 corresponds with the ML ESP at 622 (FIG. 6), the HPML M-ESPat 712 corresponds with the ML ESP at 624 (FIG. 6), and the HPML M-ESPat 716 corresponds with the ML ESP at 630 (FIG. 6).

Magnetic Cell Impedance

The present invention may be configured, according to severalembodiments, with a reference layer of high magnetic-coercivity materialthat does not change magnetic orientation (the M-ESP of the material) asthe LMC layer experiences reversal of its M-ESP. A highmagnetic-coercivity layer may be used as a reference to facilitate“reading” the data stored in the LMC layer, where the “data” is theM-ESP state of the LMC layer, e.g., e2-ESP or e1-ESP. FIG. 8 illustratesthe M-ESP of a LMC layer and a high magnetic-coercivity layer (HMC) fortwo different memory states of a typical memory cell. With reference toFIG. 8, a basic three-layer stack is illustrated at 800 for the memorycell. The stack includes LMC layer 504, tunnel junction (TJ) 502 and ahigh magnetic-coercivity layer (HMC) 802. The M-ESP of HMC 802 isusually fixed or pinned with a known magnetic orientation and does notchange during operation of the memory cell 800. The M-ESP of HMC 802 maybe fixed at e2-ESP or e1-ESP, the choice is arbitrary and does not limitthe invention.

The HMC layer 802 is typically applied to a substrate when a magneticmemory cell is constructed, therefore the HMC layer 802 is illustratedin FIG. 8 on the bottom of the stack 800, a substrate would be locatedbeneath the HMC layer 802. The TJ 502 is disposed in between the LMC 504layer and the HMC 802 layer, which places the TJ 502 beneath the LMC 504layer. The portion of the magnetic memory cell shown in 800 is meant toillustrate storing and “reading” the data stored in the LMC 504 layer.

Two memory states are shown in table 850 for the memory cell 800. Memorystate L1 at 806 occurs when the M-ESP of LMC 504 is at e2-ESP,illustrated by 808 and the M-ESP of HMC 802 is at e2-ESP, illustrated by810. The impedance is measured across the three-layer stack and is at aminimum for memory state L1. Memory state L1 occurs when the M-ESPs ofthe layers are parallel. A maximum impedance will be measured across thestack when the impedance of LMC 504 layer is at e1-ESP (816),corresponding to a second memory state L0 at 814. This condition placesthe M-ESPs of LMC 504 and HMC 802 anti-parallel. The difference inimpedance so measured provides a signal by which themagnetic-polarization state of the device maybe measured and therebyrelated to one of the two logic states.

Use of a three-layer stack as shown in FIG. 8 provides a change inimpedance of approximately 30% between the two magnetic-polarizationstates. As was previously discussed, the present giantmagneto-resistance (GMR) memory cells provide a change in impedance ofapproximately 3-10%. Thus, the present invention increases the signallevel by a factor of 3 to 10. For a given signal-to-noise requirement,this increase in signal level is proportional to a permissible reductionin volume of the memory cell, which will provide a correspondingincrease in memory density.

Alternative Energy-Gap

In an alternative embodiment of the present invention, an alternativeenergy-gap to the tunnel junction is used to increase the change inimpedance across the memory cell illustrated in FIG. 8B, which will leadto further increases in memory density as previously discussed. Inaddition, high-energy electrons may be injected via the alternativeenergy-gap into the magnetic storage layer to reverse the M-ESP of themagnetic storage layer.

With reference to. FIG. 8B, a stack 860 includes the LMC 504 layer incontact with and magnetically coupled to the first magnetic-mirror (MM)layer at 864. The first MM 864 layer is in contact with a normalconductor 862. The normal conductor 862 is in contact with a second MMlayer at 866 and the MM 866 layer is in contact with the HMC 802 layer.In one embodiment, the magnetic-mirrors 864 and 866 may be half-metalliclayers as previously described in conjunction with FIG. 6. Analternative energy-gap 870 includes MM 864, normal conductor 862 and MM866.

When the M-ESPs of the LMC 504 layer and the HMC 802 layer are parallelthe impedance between them drops to the conducting impedance of themagnetic materials and the stack. When the M-ESPs of the LMC 504 layerand the HMC 802 layers are anti-parallel, the impedance of the stackincreases significantly due to the close coupling of each MM layer withits adjacent magnetic layer, e.g., 866 with 802 and 864 with 504. Thus,each MM is aligned to the anti-aligned magnetic layers and the electronsfrom each MM are of opposite ESP and the opposing MM resists the currentflow. The aligned impedance is very low relative to the anti-alignedimpedance resulting in an increased change in impedance between the twostates.

High-energy spin-polarized electrons can be injected via the alternativeenergy-gap 870. Electrons within HMC 802 may be raised to a potential ofapproximately one (1) electron volt (eV) by electromotive forces asdescribed in conjunction with FIG. 5, thereby causing high-energy e1-SPelectrons to jump across energy-gap 870 as shown at 872 and be injectedinto the LMC 504 layer. Magnetic polarization reversal will follow aspreviously described causing the M-ESP of the LMC to reverse from e1 toe2. A subsequent reversal of the M-ESP of the LMC 504 may beaccomplished by injecting high-energy e2-SP electrons as shown at 874.High-energy e2-SP electrons 874 may be injected by duplicatingenergy-gap 870 and another high magnetic coercivity layer of e2 M-ESPlocated above the LMC layer 504 (not shown). Alternatively, high-energye2-SP electrons 874 could be generated by the tunnel junction andanother high magnetic coercivity layer of e2 M-ESP material locatedabove the LMC layer 504 (not shown). The high-energy e2-SP electrons maybe introduced according to the previous discussion directed thereto, orby any other means. The present invention is not limited thereby.

Memory Cells

The present invention may be applied to a variety of memory cells. Onesuch memory cell is illustrated in FIG. 9 as a vertical magnetic randomaccess memory (VMRAM) cell. A top view of the VMRAM cell is shown in900. In one embodiment, two sources of spin-polarized (SP) electrons areprovided by half-metallic layers (ML) 906 and 904. Several methods maybe used, that are well known in the art, to produce a source ofspin-polarized electrons. For example, a ML layer coupled with a highcoercivity magnetic layer will create close to a 100% polarization forelectrons inside of it. Alternatively, an anti-ferromagnetic layer nextto a ML layer may be used. Source 906 contains e1-SP electrons andsource 904 contains e2-SP electrons. A VMRAM cell 902 contains a LMCstorage layer as previously described. Switch 908 may be used to directe1-SP electrons from source 906 into tunnel junction 912 to place thecell in one logic state. Similarly, switch 908 may be opened and switch914 may be closed to flip the M-ESP of the LMC layer within 902 byinjecting e2-SP electrons into tunnel junction 918. For illustrativepurposes capacitor 910 is used to represent the electrical property ofthe tunnel junction 912. The same illustration for the electricalproperty of the tunnel junction is provided by capacitor 916.Alternatively, the same injection point could be used for both sourcesof SP electrons; the invention is not limited by the number of injectionpoints so arranged.

A side view 902 a of the VMRAM cell is shown in 950. A substrate 952 isshown underneath 902 a. The present invention is readily implementedwith semiconductor manufacturing processes that are well known in theart. An array of memory elements may be constructed on a monolithic chipincorporating the teachings of the present invention to create an arrayof magnetic memory cells.

The memory cell may be fashioned into round or square shapes. A rounddoughnut shape is shown in FIG. 9. The doughnut shape of the VMRAM celldirects the trapped magnetic flux into a circular magnetic domain. Thisavoids an out-of-plane magnetization in the center of the circular bitregion. 1's and 0's are encoded according to the rotational sense of themagnetization. An advantage of a round shape is that the magneticorientation is in plane and does not present an out of planesingularity. A square shape will produce one or more out of planesingularities within the magnetic field, which may be undesirable orunstable. In one embodiment, the direction of the magnetic field can becontrolled by pre-stressing the magnetic layer. Pre-stressing themagnetic layer can cause the orientation of the magnetic field to becomeout of plane, even for square shaped layers.

FIG. 10 illustrates a cross-sectional view of a vertical magnetic randomaccess memory (VMRAM) memory cell 1000. The cell 1000 may be circular orrectangular as previously discussed, but for simplicity, thisdescription will be based on rectangular cells. A conductor 1006 is alower contact to the memory cell 1000. A conductor 1007 is attached tothe top of the LMC layer 504. A Conductor 1008 is provided as a currentsource for forcing a change to the LMC 504 layer. If the M-ESPs of theHMC 802 layer and the LMC 504 layers are aligned, same magneticorientation either vertical or in plane, then the impedance through thetunnel junction 502 will be at a minimum (one memory state). If theM-ESPs of the HMC 802 layer and the LMC 504 layers are anti-aligned,then the impedance through the tunnel junction 502 will be at a maximum.

The half-metallic layers 652 and 652 a and anti-ferromagnetic layers1002 and 1002 a are optional and provide enhanced spin-polarization andspin-mirroring as previously described. The anti-ferromagnetic layers1002 and 1002 a and the half-metallic layers 652 and 652 a may be usedas desired to enhance the spin-polarization performance of the memorycell and correspondingly the design of the memory cell 1000.

In one embodiment, the HMC 802 layer is magnetized (setting the M-ESP)during manufacturing to a reference spin orientation, either e2-ESP ore1-ESP. The weak hard magnetic layer 1004 is programmed to the oppositeESP of the HMC 802 layer. The order of these is not essential, but inthe preferred embodiment the HMC 802 layer is programmed first by astrong external magnetic field since the field strength required toprogram this is the stronger of the two. The weak hard magnetic layer1004 is programmed anti-parallel to the HMC 802 layer by a weakerexternal magnetic field that will not alter the current state of the HMC802 layer. The relative magnetic moments of these two layers can bereversed without changing the basic behavior of the cells, nor is therelimitation placed on the invention, thereby.

To reset the M-ESP of the LMC 504 layer to be parallel with the M-ESP ofthe HMC 802 layer, an electron flow is introduced from the conductor1006 to the conductor 1008. To set the soft storage magnetic layer 504to be anti-parallel with the M-ESP of the HMC 802 layer, an electronflow is introduced from the conductor 1008 to the conductor 1006. Thereversal of the M-ESP of the LMC 504 layer is caused by the mechanismsarticulated above including the gain mechanisms associated with themagnetic-mirror, the half-metallic, and anti-ferromagnetic layers.

The impedance of the cell 1000 is sensed between the conductor 1007 andthe conductor 1006. The conductor 1008 is used during the cell writingoperations and is in a high impedance state relative to the conductor1007 during cell sensing.

Thus, a novel method and apparatus for reversing theelectron-spin-polarization within a material is described. Although theinvention is described herein with reference to specific preferredembodiments, many modifications therein will readily occur to those ofordinary skill in the art. Accordingly, all such variations andmodifications are included within the intended scope of the invention asdefined by the following claims.

1. An energy-gap apparatus comprising: a first magnetic mirror (MM); asecond magnetic mirror (MM); and a conductive layer of material disposedbetween said first MM and said second MM to magnetically decouple saidfirst MM from said second MM.
 2. The apparatus of claim 1, wherein saidfirst MM is a half-metallic.
 3. The apparatus of claim 1, wherein saidsecond MM is a half-metallic.
 4. The apparatus of claim 1, furthercomprising: a low magnetic-coercivity layer of material (LMC layer),having a majority electron-spin-polarization (M-ESP), said first MMhaving an ESP parallel to said M-ESP or said LMC layer, wherein saidfirst MM is configured to substantially allow electrons having an ESPparallel to said ESP of said first MM to pass through said first MM andto substantially prevent electrons having an ESP anti-parallel to saidESP of said first MM (anti-parallel electrons) from passing through saidfirst MM, said first MM coupled with said LMC layer; and a highmagnetic-coercivity layer of material (HMC layer), having a M-ESP, saidsecond MM having an ESP parallel to said M-ESP of said HMC layer,wherein said second MM is configured to substantially allow electronshaving an ESP parallel to said ESP of said second MM to pass throughsaid second MM and to substantially prevent electrons having an ESPanti-parallel to said ESP of said second MM from passing through saidsecond MM, said second MM coupled with said HMC layer.
 5. The apparatusof claim 4, wherein said first MM is configured to cause an accumulationof the anti-parallel electrons to interact with and change said M-ESP ofsaid LMC layer.
 6. The apparatus of claim 5, wherein said ESP of saidfirst MM is configured to change with said M-ESP of said LMC layer. 7.The apparatus of claim 4, wherein a first impedance may be measuredbetween said LMC layer and said HMC layer when said M-ESP of said LMClayer and said M-ESP of said HMC layer are parallel.
 8. The apparatus ofclaim 7, wherein a second impedance may be measured between said LMClayer and said HMC layer when said M-ESP of said LMC layer and saidM-ESP of said HMC layer are anti-parallel.
 9. The apparatus of claim 8,wherein said second impedance is larger than said first impedance. 10.The apparatus of claim 8, wherein said first impedance corresponds witha first memory state of a memory cell and said second impedancecorresponds with a second memory state of said memory cell.
 11. Theapparatus of claim 1, further comprising: a low magnetic-coercivitylayer of material (LMC layer), having a majority electron spinpolarization (M-ESP) parallel with an electron-spin-polarization (ESP)of said first MM; and a high magnetic coercivity layer of material (HMClayer), having a fixed M-ESP, said second MM having an ESP parallel withsaid fixed M-ESP of said HMC layer.
 12. The apparatus of claim 11,wherein a flow of spin-polarized electrons having an ESP anti-parallelto said M-ESP of said LMC layer may be injected from said HMC layer tosaid LMC layer to cause an accumulation of the spin-polarized electronsto interact with and change said M-ESP of said LMC layer.
 13. Theapparatus of claim 12, wherein said ESP of said first MM is configuredto change with said M-ESP of said LMC layer.
 14. A method, comprising:providing a first magnetic mirror (MM); providing a second magneticmirror (MM); and magnetically decoupling the first MM from the second MMwith a conductive layer of material disposed between the first MM andthe second MM.
 15. The method of claim 14, wherein said first MM is ahalf-metallic.
 16. The method of claim 14, wherein said second MM is ahalf-metallic.
 17. The method of claim 14, further comprising: couplingthe first MM with a low magnetic-coercivity layer of material (LMClayer) having a majority electron-spin-polarization (M-ESP), the firstMM having an ESP parallel to said M-ESP of the LMC layer, wherein thefirst MM is configured to substantially allow electrons having an ESPparallel to said ESP of the first MM to pass through the first MM and tosubstantially prevent electrons having an ESP anti-parallel to the ESPof the first MM (anti-parallel electrons) from passing through the firstMM; and coupling the second MM with a high magnetic-coercivity layer ofmaterial (HMC layer), having a M-ESP, the second MM having an ESPparallel to the M-ESP of the HMC layer, wherein the second MM isconfigured to substantially allow electrons having an ESP parallel tothe ESP of the second MM to pass through the second MM and tosubstantially prevent electrons having an ESP anti-parallel to the ESPof the second MM from passing through the second MM.
 18. The method ofclaim 17, wherein the first MM is configured to cause an accumulation ofthe anti-parallel electrons to interact with and change the M-ESP of theLMC layer.
 19. The method of claim 18, wherein the ESP of the first MMis configured to change with the M-ESP of the LMC layer.
 20. The methodof claim 17, further comprising: measuring a first impedance between theLMC layer and the HMC layer when the M-ESP of the LMC layer and theM-ESP of the HMC layer are parallel.
 21. The method of claim 20, furthercomprising: measuring a second impedance between the LMC layer and theHMC layer when the M-ESP of the LMC layer and the M-ESP of the HMC layerare anti-parallel.
 22. The method of claim 21, wherein the secondimpedance is larger than the first impedance.
 23. The method of claim21, wherein the first impedance corresponds with a first memory state ofa memory cell and the second impedance corresponds with a second memorystate of the memory cell.
 24. The method of claim 14, furthercomprising: providing a low magnetic-coercivity layer of material (LMClayer), having a majority electron-spin-polarization (M-ESP) parallelwith an electron-spin-polarization (ESP) of the first MM; and providinga high magnetic coercivity layer of material (HMC layer), having a fixedM-ESP, the second MM having an ESP parallel with the fixed M-ESP of theHMC layer.
 25. The method of claim 24, further comprising: injecting aflow of spin-polarized electrons having an ESP anti-parallel to theM-ESP of the LMC layer from the HMC layer to the LMC layer to cause anaccumulation of the spin-polarized electrons to interact with and changethe M-ESP of the LMC layer.
 26. The method of claim 25, wherein the ESPof the first MM is configured to change with the M-ESP of the LMC layer.27. A system, comprising: a processor; and a magnetic random accessmemory (MRAM) coupled with the processor, the MRAM comprising: a firstmagnetic mirror (MM); a second magnetic mirror (MM); and a conductivelayer of material disposed between the first MM and the second MM tomagnetically decouple the first MM from the second MM.
 28. The system ofclaim 27, further comprising: an MRAM controller coupled with theprocessor and the MRAM, wherein the MRAM controller is configured tostore data in the MRAM and to read data from the MRAM.
 29. The system ofclaim 28, further comprising: a system bus coupled with the processor;and a display coupled with the system bus.
 30. The system of claim 27,further comprising: a low magnetic-coercivity layer of material (LMClayer) coupled to the first MM, the LMC layer having a majorityelectron-spin-polarization (M-ESP) parallel with anelectron-spin-polarization (ESP) of the first MM; and a high magneticcoercivity layer of material (HMC layer) coupled to the second MM, theHMC layer having a fixed M-ESP, the second MM having an ESP parallelwith the fixed M-ESP of the HMC layer.
 31. The system of claim 30,wherein a flow of spin-polarized electrons having an ESP anti-parallelto the M-ESP of the LMC layer may be injected from the HMC layer to theLMC layer to cause an accumulation of the spin-polarized electrons tointeract with and change the M-ESP of the LMC layer.
 32. The system ofclaim 30, wherein the first MM is configured to substantially allowelectrons having an ESP parallel to the ESP of the first MM to passthrough the first MM and to substantially prevent electrons having anESP anti-parallel to the ESP of the first MM (anti-parallel electrons)from passing through the first MM. and wherein the second MM isconfigured to substantially allow electrons having an ESP parallel tothe ESP of the second MM to pass through the second MM and tosubstantially prevent electrons having an ESP anti-parallel to the ESPof the second MM from passing through the second MM.
 33. The system ofclaim 30, wherein a first impedance may be measured between the LMClayer and the HMC layer when the M-ESP of the LMC layer and the M-ESP ofthe HMC layer are parallel.
 34. The system of claim 33, wherein a secondimpedance may be measured between the LMC layer and the HMC layer whenthe M-ESP of the LMC layer and the M-ESP of the HMC layer areanti-parallel.
 35. The system of claim 34, wherein the first impedancecorresponds with a first memory state of a memory cell in the MRAM andthe second impedance corresponds with a second memory state of thememory cell.